The present invention relates to an electric field measuring apparatus for optically measuring a voltage at a portion of a sample such as a semiconductor integrated circuit device which opposes an optical probe head having an electro-optic material and, more particularly, to an electric field measuring apparatus which realizes an increase in measuring sensitivity and an improvement of operability with a simple mechanism.
In a conventional electric field measuring apparatus, an optical probe head having an electro-optic material such as an LiTaO.sub.3 crystal is used. After the position of the optical probe head is adjusted such that a sample comes very close to the electro-optic material, a specific polarized beam such as a linearly, circularly, or elliptically polarized light component is radiated on an end portion of the electro-optic material which is opposite to the side of the sample. A polarized beam reflected by the opposing end face (i.e., an end face opposite to the sample) of the electro-optic material is converted into a change in light intensity by an analyzer. A photodetector measures this change in light intensity.
The electro-optic material has electro-optic characteristics for changing polarization characteristics in accordance with the intensity of an external electric field. For this reason, the polarized beam reflected by the opposing end face of the electro-optic material is polarized in accordance with the intensity of the electric field from the sample and is then incident on the analyzer. The output level of the photodetector is equivalent to the voltage level of the sample.
The electric field measuring apparatus thus has an excellent function capable of indirectly measuring the voltage of the sample without being brought into direct contact with the sample. For example, while a semiconductor integrated circuit device is actually being operated, voltages and the like of the respective portions of internal circuit wiring can be measured without adversely affecting the operation of the semiconductor integrated circuit device. The voltage distribution of the semiconductor integrated circuit device can be examined to find an abnormal circuit portion or the like.
Conventional electric field measuring apparatuses of this type are disclosed in Japanese Patent Laid-Open No. 3-18780; U.S. Pat. Nos. 4,618,819, 4,446,425, and 4,996,475; BRIAN H. KOLNER et al., "Electro-optic Sampling in GaAs Integrated circuits", IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-22, NO. 1, JANUARY 1986, A. VALDMANIS et al., "Subpicosecond Electro-optic Sampling: Principles and Applications", IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-22, NO. 1, JANUARY 1986 and T. NAGATSUMA et al., "Subpicosecond Sampling using noncontact electro-optic probe", J. Appi. Phys. 66(9), 1 Nov. 1989.
To increase the detection sensitivity of an electric field measuring apparatus, the distance between a sample and the opposing end face of the electro-optic material in the optical probe head must be sufficiently small. When the distance is reduced, the electro-optic material is more easily influenced by an electric field from the sample. Therefore, a lower electric field intensity can be detected, and the detection sensitivity can be increased. A means for decreasing this distance has been conventionally implemented while preventing damage caused by a force acting upon contact between the electro-optic material and the sample.
In a means disclosed in Japanese Patent Laid-Open No. 3-18780, a spring having a predetermined length is arranged at one end of an optical probe. When an electro-optic material is brought into contact with a sample, the spring is brought into elastic contact with one end of the sample, thereby preventing mutual damage caused by contact between the electro-optic material and the sample beforehand. However, when a spring having a large reaction is used, a sample having a smaller strength than the reaction is damaged. A problem as to selection of a spring to be used is left unsolved. This means is not a versatile means.
In T. NAGATSUMA et al., "Subpicosecond Sampling using noncontact electro-optic probe", J. Appi. Phys. 66(9), 1 Nov. 1989, there is provided a noncontact type electric field measuring apparatus in which the distance between an electro-optic material and a sample is measured with high precision, and the electro-optic material is always kept separated from the sample.
An arrangement of this noncontact electric field measuring apparatus will be described below with reference to FIG. 30.
A half mirror 2, a multi-focus (double-focus) lens 4 having two focal points, and an electro-optic material 10 are arranged on the same optical axis. This apparatus also includes a sample stage 22 for supporting a sample 18 to oppose the electro-optic material 10.
The electro-optic material 10 is fixed to the open distal end of an optical probe head 8 fixed to a support 6. When the support 6 is vertically displaced by a piezoelectric driving unit 14, the distance between the multi-focus lens 4 and the electro-optic material 10 changes.
A light beam L containing components having two wavelengths .lambda..sub.1 and .lambda..sub.2 is incident on the multi-focus (double-focus) lens 4 through the half mirror 2. The light beam L is focused at different focal positions, reflected at these focal positions, and returns to the half mirror 2 because the multi-focus lens 4 has focal lengths f.sub.1 and f.sub.2 respectively for the wavelengths .lambda..sub.1 and .lambda..sub.2. The reflected beam is picked up by a TV camera 24, visualized on a monitor 26, and analyzed by a video signal analyzer 28.
The video signal analyzer 28 analyzes data of the reflected light beam to determine whether the incident light beam L is focused on the electro-optic material 10 and the sample 18. When the light component having the wavelength .lambda..sub.2 is determined not to be focused at the focal point corresponding to the focal length f.sub.2 (i.e., a near or far-focus state is set), the video signal analyzer 28 commands a controller 12 to drive the piezoelectric driving unit 14. The support 6 is vertically displaced, as indicated by a double-headed arrow, to perform automatic adjustment so as to obtain an in-focus state.
When the light component having the wavelength .lambda..sub.1 is determined not to be focused at a focal point corresponding to the focal length f.sub.1, the video signal analyzer 28 commands a stage controller 20 to vertically displace the sample stage 22, as indicated by a double-headed arrow. Automatic adjustment is thus performed to focus the light component having the wavelength .lambda..sub.1 on the surface of the electro-optic material 10.
As described above, when the light components having the wavelengths .lambda..sub.1 and .lambda..sub.2 are set in the in-focus state, a measurement distance h.sub.0 between the electro-optic material 10 and the sample 18 becomes a difference (f.sub.1 -f.sub.2) between the focal lengths. For this reason, this difference (f.sub.1 -f.sub.2) between the focal lengths is defined as a reference distance in advance. To actually measure the electric-field strength of the sample 18, an optical displacement sensor 16 sequentially measures the positions of the support 6 as displacement amounts from the reference distance (f.sub.1 -f.sub.2), thereby detecting the actual measurement distance h.sub.0. In addition, the distance between the electro-optic material 10 and the sample 18 can be finely adjusted so as not to contact each other because the actual measurement distance h.sub.0 can be measured.
Another non-contact electric field measuring apparatus for measuring and controlling the measurement distance h.sub.0 is also disclosed in a reference (Singaku Giho Vol. 91, No. 234, p. 33). This apparatus measures the distance between a sample and an optical probe using a balance mechanism. In addition, the displacement of the optical probe is also measured using an optical displacement sensor.